Single Stage or 2 Stage

johnboy70_99March 15, 2012

I am replacing my upstairs furnace and AC unit in my 2500sq ft house. Sizing of the old unit was OK - question is should I go 2 stage vs 1 Stage. I am in Massachusetts. We are willing to pay the extra $$ upfront for the 2 stage to get the extra comfort and quieter operation, I realize that we will not make back the difference in savings and that's OK.

What I am concerned about is the reliability of the 2 stage equipment, it is more parts to break. Also looking at lennox vs. American Standard. We have gotten several quotes from different contractors and understand that the installation is more important than the equipment itself, hopefully we have found companies that will do the installation well.

Has anyone seen 2 stage units having a history of more repair problems vs. single stage?

My 2 stage has completed its 4th cooling system with nary a problem. During its spring maintenance, the tech found the capacitor to be a little low, and according to the company's standard, it was replaced, thus averting a failure in future years.

You didn't state what size your current unit is. As 2 stage units come in whole ton sizes, and your unit happens to be a 'half-ton' size, don't fall for the 'well it will run mostly in first stage, so it will be economical to run', or 'it will dehumidify better'. Two stage/multi-stage equipment is for comfort, not economy.

An accurate heat calc (manual J) should be performed to determine your actual (size) needs. Did your unit cycle frequently during the hottest weather?

I don't think 2-stage AC condensers have been in the field long enough to demonstrate their reliability versus a single stage unit. My 2-stage ACs have gone through three summers with no problems so far.

I agree the more complicated the machine is, the higher the probability of a problem. But here is a counter argument. A single stage AC always operates at its highest capacity. Many times it is oversized for mild temperature conditions and will cycle on and off frequently. The constant starting and stopping is stressful to the components. A 2-stage unit operating on the low stage will cycle on and off less frequently under the same conditions. I would think this would result in less wear and tear and potentially higher reliability.

I have talked to 6 contractors so far and they all agree on 2.5 ton. They did take measurements of the rooms, asked about the quality of the insulation. Some looked at the size of all the return air ducts, etc.

I am curious about how accurate a manual J calculation is vs. the observation of the performance of the current system. When they asked how the house was insulated, I said average, kind of guessing. Wouldn't that influence the sizing a bit? Knowing that we have a 2.5 ton system now and it was working good for 10 years, would that be as good of a judge? Although all the contractors asked about the insulation and measured things, I think some of them did base their judgement by the performance of the current system.

One said that I could try to go down to 2 ton which may be undersized a bit but would probably work pretty well, it might not be able to hold temperature on the hottest of days but probably would be OK.

The contractors that have looked at the ductwork so far have said it was OK except returns are not enough. They calculated 850CFM capacity out of the needed 1000, I will be adding one return to get the additional sizing needed.

The deal breaker for me is probably the lack of a 2.5 ton 2 stage unit. I checked the quote and it looks like they were going to give me a 3ton 2 stage unit, which would be close to right sized when running only 1 stage, and too much when running both compressors and high speed. I am a bit disappointed they did not point this out to me.

I am leaning towards the single stage 2.5 ton unit from another contractor - he put the system in my old house and did a great job, he seems to do very high quality work. Only reason I was on the fence and didn't go with his suggestion was I wanted 2 stage and he suggested against it. He said it wasn't worth it. Sounds like he was right. The fact that it is 3K cheaper for single stage does not really hurt either. Does that sound like the way to go?

I think historical observations on sizing is very important. In an older house, it blows away a manual J. Your window SHGC trumps everything else and that may be a number you have to guess at.

I think you are probably right to go with the 2.5 single. Obviously, even with a 2 stage 3 ton, you will get better dehumidification. You will also be able to cool down faster and be able to handle those rare 100 degree days. It will also be quieter with less airflow the vast majority of the time.

I think the units are considering run the first stage at 66% of max or 2 tons in your case. You might consider Carrier which has a single stage at 50% of max. That would really make a difference in your case.

But in MA, I'm not sure I would spend the $3000 despite the advantages. And Carrier will cost even more.

What is the area of the upper floor? Just because a contractor has measured the area means he has done a Manual J calculation. Have you seen a print out?

My upper floor has an area of about 1650 square feet. The attic has R-30 insulation, walls are R-13, the windows are double pane. The back of the house has six windows facing the south. The cooling load calculation came out to 2.4 tons. I am in central NJ. I went from a 2 ton single stage unit, to a 3 ton 2-stage. It works great in keeping the humidity below 40% in the summer.

You said the current unit barely keeps up in the summer. Can you decribe what you mean by this?

It is nice to have a little extra capacity in order to set back the temperature during the day when no one is in the bedrooms. In the evening the the high stage comes on to cool down the upper level quickly.

I am not sure. It runs so quietly I have to put my hand over the vents to figure out if it is in low or high stage. Since I set it back during the heat of the day, I would estimate not much. I set the upstairs temperature to 78-79 degrees in the evening. The low humidity makes if feel comfortable for me.

To clarify - the current unit appeared to be properly sized as it was able to hold temperature on the 95-100 degree days by running pretty much all day. It seems like it was close to correctly sized.

Maybe one factor is we have many South facing windows and no trees shading in the summer. The first floor is all bay and large picture windows, and the house runs east/west so the long side of the house faces south. They are fairly insulated but I am guessing that factors in the cooling requirements? A trade off for all of the great natural light we get.